Neuromuscular diseases encompass a large collection of disorders, ranging from relatively mild conditions such as focal compression neuropathies and nerve root injuries, to severe and life-threatening syndromes, including amyotrophic lateral sclerosis (ALS) and muscular dystrophies. These disorders may lead to muscle atrophy and weakness, caused either by injury to or disease of the neuron (neurogenic disorders), the neuromuscular junction, or the muscle cell itself (myopathic disorders). Another disorder, disuse atrophy, may occur when a limb is immobilized or a patient is bed-bound for a prolonged period of time, although not classically considered a neuromuscular disorder, also produces substantial morbidity.
Neuromuscular diseases have been assessed and diagnosed using various techniques, including nerve condition studies, needle electromyography, muscle imaging, muscle biopsy and genetic testing. However, the initial assessment of the neuromuscular diseases has advanced relatively little beyond conventional needle electromyography and nerve conduction techniques. Similarly, there have been few good approaches to the assessment of disuse atrophy and dysfunction.
Nerve conduction studies (NCSs) and needle electromyography (EMG) are often the first tests obtained when evaluating a patient for neuromuscular causes of atrophy. NCSs involve stimulation of a nerve with one set of electrodes and recording the resulting muscle or nerve potential with a second set of electrodes. Although useful for evaluating nerve pathology, NCSs are of limited Use for evaluating muscle disease or disuse states. The stimuli can be uncomfortable and only a relatively limited set of distal muscles in the arms and legs can be evaluated.
Needle electromyography is geared more specifically to muscle evaluation. Needle electromyography can provide a quick survey of muscles to determine whether they are being affected by neurogenic injury or myopathic injury. However, the test has considerable limitations. First, needle electromyography is very subjective because physicians qualitatively assess the attributes of motor unit action potentials (MUAPs) as they rapidly pass across an oscilloscopic display. Second, there are substantial limitations with respect to the sensitivity of needle electromyography. It is a common experience amongst electromyographers that only with extensive probing are one or two questionably abnormal MUAPs identified. Third, the lack of quantifiable results makes EMG an unsuitable modality for following disease progression/remission. Finally, needle EMG remains a somewhat painful, invasive procedure and can thus only be used in a very limited fashion in children.
Imaging techniques such as magnetic resonance imaging (MRI) and ultrasound have found some use in muscle atrophy assessment. For example, MRI can be used to identify muscles with active inflammation to assist with biopsy site choice in patients with myositis. However, MRI has otherwise remained of limited use since it is difficult to evaluate different areas of the body, is costly, cannot easily assess dynamic muscle states during muscle contraction, and may not be used in patients with pacemakers and implanted defibrillators. Ultrasound has found limited use in neuromuscular disease and disuse atrophy assessment, and remains very qualitative.
Muscle biopsy is another test for evaluation of muscle disease and can be helpful in arriving at a specific diagnosis. Muscle biopsy frequently yields limited or contradictory information and may be unsuitable for monitoring progression of atrophy because of its inherent invasiveness. Given that many diseases are patchy (i.e., regions of diseased muscle tissue is interspersed throughout ostensibly healthy muscle tissue), a negative biopsy does not exclude disease, and repeat biopsies sometimes need to be performed.
Genetic tests can be very useful for assisting in the evaluation of a number of mostly rare conditions (such as the muscular dystrophies), but is expensive and not relevant to a variety of the most common, acquired conditions.